Charged particle beam systems are used in a variety of applications, including the manufacturing, repair, and inspection of miniature devices, such as integrated circuits, magnetic recording heads, and photolithography masks. Charged particle beams include ion beams and electron beams.
Ions in a focused beam typically have sufficient momentum to micromachine by physically ejecting material from a surface. Because electrons are much lighter than ions, electron beams are typically limited to removing material by inducing a chemical reaction between an etchant vapor and the substrate. Both ion beams and electron beams can be used to image a surface at a greater magnification and higher resolution than can be achieved by the best optical microscopes.
Ion beam systems using gallium liquid metal ion sources (LMIS) are widely used in manufacturing operations because of their ability to image, mill, deposit, and analyze with great precision. Ion columns in focused ion beam (FIB) systems using gallium LMIS, for example, can provide five to seven nanometers of lateral resolution. Because ion beams tend to damage sample surfaces even when used to image, ion beam columns are often combined with electron beam columns in dual beam systems. Such systems often include a scanning electron microscope (SEM) that can provide a high-resolution image with minimal damage to the target, and an ion beam system, such as a focused or shaped beam system, that can be used to alter workpieces and to form images. Dual beam systems including an LMIS FIB and an electron beam are well known. For example, such systems include the Quanta 3D FEG™ System, available from FEI Company of Hillsboro, Oreg., the assignee of the present invention. The ion beam can be used, for example, to cut a trench in an integrated circuit, and then the electron beam can be used to form an image of the exposed trench wall.
Unfortunately, high-precision milling or sample removal often requires some tradeoffs. The processing rate of the LMIS FIB is limited by the current in the beam. As the current is increased, it is harder to focus the beam into a small spot. Lower beam currents allow higher resolution, but result in lower milling rates and hence longer processing times in production applications and in laboratories. As the processing rate is increased by increasing the beam current, the processing precision is decreased.
Further, even at higher beam currents, focused ion beam milling may still be unacceptably slow for some micromachining applications. Other techniques, such as machining with a femtosecond laser can also be used for faster material removal but the resolution of these techniques is much lower than a typical LMIS FIB system. Lasers are typically capable of supplying energy to a substrate at a much higher rate than charged particle beams, and so lasers typically have much higher material removal rates (typically up to 7×106 μm3/s for a 1 Watt laser operating at al kHz laser pulse repetition rate) than charged particle beams (typically 0.1 to 3.0 μm3/s for a gallium LMIS FIB). Laser systems use several different mechanisms for micromachining, including laser ablation, in which energy supplied rapidly to a small volume causes atoms to be explosively expelled from the substrate. All such methods for rapid removal of material from a substrate using a laser beam will be collectively referred to herein as laser beam machining.
FIG. 1 is a schematic illustration 10 of a prior art laser ablating a surface. When a high power pulsed laser 12 producing laser beam 13 is focused onto a target material 14 supported by a stage 15 and the laser fluence exceeds the ablation threshold value for the material, chemical bonds in the target material are broken and the material is fractured into energetic fragments, typically a mixture of neutral atoms, molecules, and ions, creating a plasma plume 16 above the material surface. Since the material leaves the reaction zone as an energetic plasma, gas, and solid debris mixture, the ablation process resembles explosive evaporation of the material that propels material fragments 18 up and away from the point where the laser beam 13 is focused.
As compared to charged particle beam processing, laser ablation is capable of removing a relatively massive amount of material very quickly, with material removal rates more than 106× faster than a Ga LMIS FIB. The wavelength of light in the laser beam, however, is much larger than the wavelength of the charged particles in the charged particle beams. Because the size to which a beam can be focused is, in part, limited by the beam wavelength, the minimum spot size of a laser beam is typically larger than the minimum spot size of a charged particle beam. Thus, while a charged particle beam typically has greater resolution than a laser beam and can micromachine extremely small structures, the beam current is limited and the micromachining operation can be unacceptably slow. Laser micromachining, on the other hand, is generally much faster, but has poorer resolution due to diffraction.
Unfortunately, laser ablation, because of the large volume of debris produced, can tend to result in the gradual coating of the laser objective lens with debris, which ultimately degrades the performance of the laser. For laser ablation performed at atmosphere, a gas is typically blown across the surface to keep the debris away from the lens. Also, replaceable glass cover slips are sometimes used to cover the lens. Neither of these techniques can be easily used in a combined charged particle beam/laser system, however. The use of a gas is not practical because the sample chamber must be under vacuum for charged particle beam operation. Also, the use of glass covers is impractical; both because it requires venting the sample chamber to atmosphere to replace the covers and also because glass suffers from a very undesirable charging effect that will degrade the operation of the charged particle beam, especially for imaging purposes.
What is needed is a method and apparatus for protecting the laser optics from the build-up of debris during laser ablation in a charged particle system and preventing the resulting gradual degradation of the laser beam intensity.